Fluidized-bed reactors are useful in a large number of applications in the chemical-process and heat-exchange fields and commonly owned prior U.S. Pat. Nos. 3,565,408 and 3,579,616 disclose various processes which can make use of heat-exchange and reactor capabilities of a fluidized bed.
Endothermic processes can be carried out in an orthodox or conventional fluidized bed, which is definable as a fluidized bed having a discontinuity in the solids gradient within the reactor or a well-defined separation between a gas zone above the bed and the particle-containing bed itself.
In the orthodox fluid bed, fluidization is carried out from below the particle layer so that the particles move within a bed or zone having upper and lower boundaries, the dense phase of the fluidized bed being separated from an overlying gas or dust space by a boundary layer having a high density gradient or a discontinuous density gradient. Orthodox fluidized beds of this type are described in British Pat. No. 878,827 and U.S. Pat. No. 2,799,558, for example.
Endothermic processes may also use a co-called "expanded" or "fast" fluidized bed which generally has no sharp density gradient separating a high-density particle phase from a lower-density gas phase.
In the fast fluidized bed, fluidization is carried out so that there is no well-defined upper boundary layer. In other words, solids density (solid volume or mass per unit of reactor volume) gradually diminishes from the bottom of the bed toward the top thereof. In order to achieve a fast expanded fluidized bed of this type, one supplies the fluidizing gas at a much higher velocity than is necessary to maintain the orthodox fluidized bed, the velocity being sufficient to enable the gas to entrain substantial portions of the solids of the bed out of the reactor. This of course would decrease the quantity of solids in the reactor and hence the solids content of the reactor must be replenished. The solids concentration or density (particles/unit volume or solid mass/unit volume) of an expanded or fast fluidized bed is thus less than the solids concentration of an orthodox fluidized bed but is greater than the solids concentration in the gas zone above the orthodox fluidized bed.
As described inter alia in the aforementioned commonly owned U.S. patents, in which some of us appear as inventors when fine-grained alumina hydrate is calcined, the gas velocity may correspond to a gas rate of 1500 to 3000 standard cubic meters per square meter of reactor cross section per hour (Nm.sup.3 /m.sup.2 h) and the density of the suspension may be maintained above 30 kilograms per cubic meter (kg/m.sup.3) on the average throughout the top of the reaction zone and 100 to 300 Kg/m.sup.3 at the lower part of the reaction zone.
The solids entrained with the gas are separated out in, for example, a cyclone-type separator and part of the solids may be recirculated into the fluidized bed while the remainder constitutes a desired product. For further background on this type of system, reference is made to German printed application (Auslegeschrift) No. 1,146,041.
In the production of alumina it is also known to use a fast fluidized bed in which solids are separated from the gas and are recycled to the fluidized bed to supply heat to the latter. At least part of the heat requirement is met by feeding hot gases into the fluidized bed in an enlarged portion of the shaft above the gas distributor and above the inlet for recycled solids. Reference is made, in this connection, to German Pat. No. 1,092,889.
All of these methods have undesirable heat-utilization characteristics.
Furthermore, in practice it is found to be difficult with these conventional systems to insure uniform combustion of the fuel without overheating. The problem is encountered both when the combustion takes place within or adjacent the fluidized-bed reactor and also when it takes place in a separate combustion chamber spaced from the reactor. For high-temperature processes, combustion of the fuel in a combustion chamber outside the fluidized-bed reactor makes it difficult to provide materials capable of withstanding the high combustion temperature in an economical or convenient manner.
An attempt to solve this problem has been made, as described in U.S. Pat. No. 3,579,616, by utilizing the waste heat of the exhaust gas and the entrained solids to thereby increase the utilization of fuel and achieve an optimum heat-consumption rate. The combustion was carried out in two stages, first in a high-density region only with fluidizing air in substoichiometric amounts, then in a subsequent stage in the presence of secondary air to achieve at least stoichiometric combustion (i.e. stoichiometric or slightly over-stoichiometric combustion).
An advantage of this system was the elimination of overheating of limited regions of the fluidized bed and the ability to maintain substantially constant temperature conditions and accurate temperature control. However, the system has been found to have a technological disadvantage. If long minimum residence time is required for the solids in certain reactions, the reactor must be increased considerably in height and the result is a considerable pressure loss as well as increased energy consumption in displacing the gas through the reactor.
It should be noted that other alternatives are equally unsatisfactory. For example, the decreased residence time means that the reaction does not reach completion or a sufficient degree of completion while greater bed densities can only be achieved with lower gas velocities and high pressure drop. For a given density of the suspension and pressure drop in the fluidized bed, therefore, there is a decreased rate of production and for an increased production rate the density of the suspension must be increased so that the pressure drop will also increase and high energy requirements are a result.
Furthermore, solutions which lead to greater circulation rates of the solids than is otherwise technologically required also lead to unnecessary energy consumption.